The Hindu Lunar Months, Chaitra to Phalguna, and How They Get Their Names

The Hindu lunar year has twelve months: Chaitra, Vaishakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashvina, Kartika, Margashirsha, Pausha, Magha and Phalguna. Each name comes from a nakshatra (lunar star-segment) near which that month's full moon traditionally falls: Chaitra from Chitra, Vaishakha from Vishakha, and so on. In practice, though, panchangs decide which month is which by the Sun. A lunar month takes the name of the solar sign the Sun is in when that month's new moon occurs. The star link explains the names; the solar rule assigns them.

The twelve months

Rough Gregorian spans below are for the new-moon-ending (amanta) month and shift by a couple of weeks from year to year. They are a guide to the season, not dates.

#	Sanskrit name	Common everyday form (Hindi and related languages)	Namesake nakshatra	Sun's sign at the month's new moon	Roughly
1	Chaitra	Chait	Chitra	Mina (Pisces)	March–April
2	Vaishakha	Baisakh	Vishakha	Mesha (Aries)	April–May
3	Jyeshtha	Jeth	Jyeshtha	Vrishabha (Taurus)	May–June
4	Ashadha	Asadh	Purva/Uttara Ashadha	Mithuna (Gemini)	June–July
5	Shravana	Sawan	Shravana	Karka (Cancer)	July–August
6	Bhadrapada	Bhadon	Purva/Uttara Bhadrapada	Simha (Leo)	August–September
7	Ashvina	Ashwin, Kuar	Ashvini	Kanya (Virgo)	September–October
8	Kartika	Kartik	Krittika	Tula (Libra)	October–November
9	Margashirsha	Agahan (Agrahayana)	Mrigashira	Vrishchika (Scorpio)	November–December
10	Pausha	Poos	Pushya	Dhanu (Sagittarius)	December–January
11	Magha	Magh	Magha	Makara (Capricorn)	January–February
12	Phalguna	Phagun	Purva/Uttara Phalguni	Kumbha (Aquarius)	February–March

The signs are sidereal (measured against the stars), which is why they sit about three and a half weeks later than the Western zodiac dates many people know.

Layer one: where the names came from

The month names are adjectives formed from nakshatra names. Chaitra means "of Chitra", Kartika "of Krittika", Pausha "of Pushya". The idea, found in the older calendar tradition, is that the full moon of each month shines in or near that star group. Opposite the Sun at full moon, the Moon "marks" the month.

Three of the namesake stars come in pairs (Purva and Uttara Ashadha, Bhadrapada and Phalguni), and each pair shares one month. Twelve months from 27 nakshatras means some nakshatras, such as Rohini, Ardra and Revati, never give their name to a month.

Many panchangs add two further associations: a presiding deity for each month, and the six seasons (ritu) paired with the months. Like the star link itself, these are traditional associations, not astronomical requirements.

Layer two: how a panchang actually names the month

A calendar needs a rule it can compute, and the star link is too loose for that. The rule used across the classical system is solar. As the astronomer S. K. Chatterjee summarises it, each lunar month is named after the solar month in which its opening new moon occurs.

Step by step:

1. Find the moment of new moon that begins the month.
2. Check which sidereal sign the Sun is in at that moment.
3. Give the lunar month the name tied to that solar month. A new moon with the Sun in Mina starts Chaitra; with the Sun in Mesha, Vaishakha; and so on round the table.

Put the other way round: the lunar month during which the Sun enters Mesha (Mesha sankranti) is Chaitra. In an ordinary month both phrasings give the same result.

Why the full moon lands "in or next to" the namesake star

Take Chaitra. Its new moon happens while the Sun is somewhere in Mina, which spans 30°. About fifteen days later, at full moon, the Sun has moved on about 15° and the Moon is directly opposite it. Work that through and the Chaitra full moon can fall anywhere across roughly 30° of the Moon's path: in Hasta, Chitra or Swati, the namesake and its two neighbours. The same arithmetic applies to every month, although the window is not always centred on the namesake: the Shravana full moon, for example, falls more often in Dhanishtha than in Shravana itself. So if a panchang shows "Chaitra Purnima" with the Moon in Swati, nothing has gone wrong. The month was fixed by the Sun; the star name is a historical label that fits only approximately.

What happens in a year with thirteen months

Twelve lunar months are about 354 days, roughly 11 days short of a solar year. Occasionally a whole lunar month begins and ends with no change in the Sun's sign. There is then no solar month to name it after, so it takes the name of the following month with the prefix adhika ("extra"). The ordinary month that follows is called nija or shuddha. Much more rarely, a month name is skipped altogether (kshaya). Both cases have their own article: Adhik Maas and Kshaya Maas.

Same name, different fortnight: the amanta/purnimanta catch

In much of North India a month is counted from full moon to full moon (purnimanta). In much of the South and West it runs from new moon to new moon (amanta). The names are the same, but the waning fortnight carries a different month name in the two systems. A festival in "Krishna Paksha of Shravana" in one region's panchang can be the same day as "Krishna Paksha of Bhadrapada" in another's. The full explanation is in Amanta vs Purnimanta.

Solar month names are a separate system

Several regions, including Tamil Nadu, Kerala, Bengal, Odisha and Assam, also keep a solar calendar whose months follow the Sun's passage through each sign. Chatterjee notes that these solar months vary in length from about 29½ to 31½ days, because the Earth's orbit is elliptical. They begin by regional conventions about the day of the Sun's entry into a sign (sankranti).

The confusing part is that regions attach the old names to these solar months differently:

• In the Bengali solar calendar, the month that begins when the Sun enters Mesha is Boishakh (Vaishakha).
• In the Tamil calendar, the same solar month is Chithirai (Chaitra).

So a Tamil "Chithirai" and a North Indian lunar "Chaitra" are not the same span of days. Regions also differ on where the year begins: with Chaitra in much of India, but with Kartika in much of Gujarat, according to Mishra's survey. Regional new years are covered in India's regional calendars.

Finding today's month

To see which month and fortnight it is today, look at the masa (month) and paksha lines of a panchang for your city, such as your family's or temple's panchang. It is good practice to check whether the panchang you are reading is amanta or purnimanta before comparing it with another.

If a panchang does not say which it is, a second panchang will tell you during the waning fortnight. Suppose one calls the current fortnight Krishna Paksha of Shravana and the other calls it Krishna Paksha of Bhadrapada: the one giving the later month name is counting purnimanta. In the waxing fortnight the two will name the same month.